Millstone Grit Outcrops of the Hope Valley area, northeast Derbyshire, England

- a rough and tough clastic sediment!

Millstone Grit [280 kb] Millstone Grit [306 kb]

Figs. 1-2: Millstone Grit outcrops. I hope I have the names correct - the moors of central and northern England are dotted with rocky outcrops, often given strange and wonderful place names. Many appear on the relevant Ordnance Survey maps, which I have always found to be inspiring examples of the cartographic arts. These all occur within a 1x1 km area on the north side of the main A625 road that runs west from Sheffield across the Peak, and are accessed from the Surprise View car park. Left: a vantage point atop the steep quarry faces on Millstone Edge. Right: a rock atop Millstone Edge in the general area of Over Owler Tor.

The outcrops shown this month are located on and near Millstone Edge, near the village of Hathersage in the English Peak District, a few km west of the southern suburbs of the city of Sheffield. The area is scarcely 10 km (as the crow flies, i.e., in a straight line) E.S.E. of the well-known mineral and caving locality of Castleton (Ford, 1996). The Peak District includes most of the southern Pennine Hills, and lies almost entirely in the quadrilateral defined by Manchester, Sheffield, Derby and Stoke-on-Trent; most lies in Derbyshire, parts in Cheshire and Staffordshire (Cope et al., 1972).

"Rock of the Month # 266, posted for August 2023" ---

The Millstone Grit

The Millstone Grit is especially well-developed in the Peak District but also occurs along the Pennine chain and adjacent areas of central and northern England. The coarse sandstones (aka grit or gritstone) are associated with local developments of conglomerates and shales. They are attributed to deposition in a deltaic environment, in contrast to the onshore swamps of the younger Coal Measures. Cross-bedding, seen in each photo but especially in Figure 3, is a characteristic sedimentary structure of these rocks. They are very tough, composed principally of hard quartz, though even these rocks are subject to slow erosion as individual sand grains slough off under stresses of wind, water or human activity. Note that cross-bedding may indicate either aqueous (e.g., deltaic, continental margin) or subaerial (eolian, desert) environments, the distinction of these possibilities requiring careful fieldwork (Selley, 1970, pp.12-13, 52-56, 78-87). A local case study is the Kinderscout Grit sequence (ibid., pp.81-87). The Grit, local representative of the Millstone Grit, comprises over 400 feet of strata, mainly coarse, pebbly arkosic sandstones, the base displaying huge channels cut into underlying shales, often over 100 feet deep and almost 1,000 feet wide.

Beyond the Peak, the Grit also outcrops or subcrops in parts of Yorkshire and Lancashire, Staffordshire and Shropshire (see Edwards and Trotter, 1954, pp.32-41; Hains and Horton, 1969, pp.34-39) and Leicestershire (Ford, 1972). The Grit becomes thinner and more shaley north from the Peak District, across the Askrigg Block (Trueman, 1971) towards the Scottish border (Taylor et al., 1971, pp.54-59). But the Grit or its equivalents are also more widely distributed (Jukes-Browne, 1886; Geological Survey, 1957). Thus the Grit is found also in: the Bristol area and Mendip Hills (Welch, 1933; Kellaway and Welsh, 1993); south Wales (Neville George, 1970; Bassett and Bassett, 1971; Carr, 2001); southwest Wales (IGS, 1960, 1962) and Ireland (Praeger, 1937). An equivalent is the Limestone Coal Formation and associated strata of the Clackmannan Group in the Glasgow area. These are a mixture of marine, deltaic, fluvial and coal-swamp strata that succeeded the lavas and volcaniclastic rocks of the Clyde Volcanic Plateau (Allison and Webster, 2017; Webster, 2020).

The Millstone Grit is broadly conflated with a time division, the Namurian stage, third of six subdivisions of the European Carboniferous, and basal stage of the Upper Carboniferous, circa 326 to 313 Ma. It is named for a city and region in Belgium where strata of this age were described. The tough, abrasive varieties of the Grit have long been prized for their excellence as millstones (Moore, 1983). A scattering of millstones, in varied degrees of completion, lie abandoned around the nearby Toad's Mouth rock. Stratigraphically beneath the Millstone Grit is the Carboniferous Limestone, which locally hosts base-metal deposits (lead) as well as nearby concentrations of a purple, banded variant of fluorite (fluorspar), known as Blue John, sought after since Roman times (Ford, 1996; see also Blue John Fluorite on these pages).

Fossils are generally poorly preserved in sandstones, especially when viewed in comparison to limestones, where precursor calcareous muds enveloped and swaddled remains as they were buried and mineralized. Yet fossils do occur in the Millstone Grit, including cephalopods (goniatites such as Reticuloceras, Black, 1972). Marine fossils include genera such as Goniatites and the bivalve Posidonia (Wells and Kirkaldy, 1966, pp.233-247). The fauna is in contrast to the later non-marine bivalves of the younger Coal Measures.

Millstone Grit [250 kb]

Fig. 3 : A third outcrop, a rock formation displaying cross bedding in sandstone, a landmark known as Mother Cap.

Outdoor sports

The coarse grit is well known to British rock climbers. The rough surfaces can prove painfully abrasive, even as they offer excellent friction. Climbing guidebooks in the Peak are abundant, and often offer a geological summary in the introductory pages (e.g., R.D. Brown in Griffiths and Wright, 1976, pp.7-10; J. Soper in Gregory, 1978, pp.7-9; and Milburn, 1983, pp.6-10). Numerous routes have been set on both natural crags (cliffs, escarpments) and quarry faces (as at Millstone Edge and Lawrencefield, see Poucher, 1973). Millstone Edge is a popular rock climbing venue (see, e.g., Nunn, 1975, pp.46-61 and Milburn, 1983, pp.270-298). Rock climbing at Millstone Edge seems to have started slowly in the early 1920s, accelerating in the 1950s with increasing numbers of pegged (as opposed to free) routes (Gregory, 1978).

Acknowledgements: Thanks to Clare and Doug for a tour of this area, and to Carl and Frances for a guided visit to the National Trust property at Acorn Bank, west of Appleby-in-Westmorland, where water power was used to drive three wheels, respectively used to grind grain for a flourmill (millstones of grit and local volcanic rock), to power a sawmill, and to raise carts of ore from a small gypsum mine.


Allison,I and Webster,DJ (2017) A Geological Guide to the The Fossil Grove, Glasgow. Geological Society of Glasgow / Ringwood Publishing. 21pp.

Bassett,DA and Bassett,MG (editors) (1971) Geological Excursions in South Wales & The Forest of Dean. Geologists' Association Guide, South Wales Group, Cardiff, 267pp.

Black,RM (1972) The Elements of Palaeontology. Cambridge University Press, 339pp.

Carr,S (2001) A glaciological approach for the discrimination of Loch Lomond Stadial glacial landforms in the Brecon Beacons, south Wales. Proc.Geol.Assoc. 112, 253-262.

Cope,FW, Broadhurst,FM, Downie,C and Jackson,JW (1972) The Peak District. Geologists' Association Guide 26, 2nd edition, 38pp.

Edwards,W and Trotter,FM (1954) British Regional Geology: The Pennines and Adjacent Areas. Institute of Geological Sciences / HMSO, 3rd edition, 86pp.

Ford,TD (1972) Geology. In "Leicester and its Region", Leicester University Press / British Association, 19-58.

Ford,TD (1996) The Castleton Area, Derbyshire. Geologists' Association Guide 56, 94pp.

Geological Survey (1957) Geological Survey `Ten-Mile' Map, Sheet Two. GSGB, 1:625,000 scale map, England and Wales, 2nd edition.

Gregory,D (1978) Froggatt area. British Mountaineering Council, 1st edition, New Series, 200pp.

Griffiths,B and Wright,A (1976) Stanage area. British Mountaineering Council, 1st edition, New Series, volume 1, 202pp.

Hains,BA and Horton,A (1969) British Regional Geology: Central England. Institute of Geological Sciences / HMSO, 3rd edition, 142pp.

IGS (1960) Haverfordwest. IGS map, England and Wales, sheet 228, 1:63,360 scale, drift edition.

IGS (1962) Milford Haven. IGS map, England and Wales, sheet 226 & 227, 1:63,360 scale, solid edition.

Jukes-Browne,AJ (1886) The Student's Handbook of Historical Geology. George Bell and Sons, Covent Garden, London, 597pp.

Kellaway,GA and Welch,FBA (1993) Geology of the Bristol district. British Geological Survey Memoir, 200pp.

Milburn,G (1983) Stanage Millstone. British Mountaineering Council, Peak District Climbs, 4th series, volume 1, 336pp.

Moore,DT (1983) Petrological aspects of some sharpening stones, touchstones, and milling stones. In "The Petrology of Archaeological Artefacts" (Kempe,DRC and Harvey,AP editors), Clarendon Press, Oxford, 374pp., 277-300.

Neville George,T (1970) British Regional Geology. South Wales. IGS / HMSO, 3rd edition, 154pp.

Nunn,P (1975) Rock Climbing in the Peak District. Constable, London, 304pp.

Poucher,WA (1973) The Peak and Pennines, from Dovedale to Hadrian's Wall, a pictorial guide to walking in the district and to the safe ascent of its principal mountain groups. Constable & Co. Ltd., London, 2nd edition, 419pp.

Praeger,RL (1937) The Way that I Went. Collins Press Ireland, 2nd edition, xii+394pp., republished 2014.

Selley,RC (1970) Ancient Sedimentary Environments. Chapman and Hall, 237pp.

Taylor,BJ, Burgess,IC, Land,DH, Mills,AC, Smith,DB and Warren,PT (1971) British Regional Geology: Northern England. Institute of Geological Sciences / HMSO, 4th edition, 121pp.

Trueman,AE (1971) Geology and Scenery in England and Wales. Penguin Books Ltd, revision of 1949 original by Whittow,JB and Hardy,JR, 400pp.

Webster,DJ (2020) The Fossil Grove. Geological Society of Glasgow, Presentation via Zoom, Scottish Geology Festival 2020.

Welch,FBA (1933) The geological structure of the eastern Mendips. Q.J.Geol.Soc.London 89, 14-52.

Wells,AK and Kirkaldy,JF (1966) Outline of Historical Geology. Thomas Murby & Co., London, 6th edition, 533pp.

Graham Wilson, posted 13-14,16-17 July 2023

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